The rapid sequence 51 Chromium red cell survival test is now a widely accepted laboratory procedure used primarily in situations when compatible blood cannot be found by usual in vitro serologic techniques. The project, therefore, now concentrates on those patients and blood donors who have poorly understood and incompletely characterized red cell antibodies detected in their serum. Characterization of the in vivo significance of these antibodies will allow more rational selection of blood for these people should transfusion be necessary in the future. Several rare or unusual antibodies have been studied (e.g. anti- Yta, anti-H, anti-A1I, "enzyme-reactive" antibodies, etc.). Collaborative studies have been completed with the American Red Cross to compare the 51Cr test with an in-vitro macrophage effector assay. Three distinct patterns of immune red cell destruction have been confirmed using 51Cr labelled cells. These patterns can be identified when the test is carried to 24 hrs, allowing more appropriate transfusion decisions to be made.